In a packet-switching protocol, the messages are divided into packets before being sent. Each packet is then transmitted individually and may follow different routes from a source node to a destination node of the communication network. Once all the packets forming a message have arrived at the destination, they are collected to form the original message. Most communication networks are based on packet-switching techniques. The ordinary telephone service, on the other hand, is based on a circuit-switching technique in which one line is assigned to the transmission of a message between two nodes on the network. Circuit-switching is ideal when data needs to be transmitted quickly and needs to arrive in the same order as that in which it was sent. Packet-switching is more efficient and robust for data that can tolerate a delay in transmission, such as messages of the e-mail type for example.
In a communication network based on packet-switching, a packet forms the unit entity that is transmitted, during a routing step of the message, from a transmitting node of the network to a receiving node of the network via a communication link. According to the routing method, the packets may or may not be formed into queues at each node on the network, the queues preferably being of minimum possible lengths. Each packet comprises a header and data, a header comprising:                an address of the source node,        an address of the destination node,        the size of the packet, or a control word, or nothing in the case of packets of fixed size,        a message identifier, and        a packet identifier.        
However, the header itself contributes to the size of the packet. Consequently, a message that has been divided into packets takes up more space than the original message, the effect of which is to reduce the bandwidth of the communications network.
Patent application EP 1081910 describes a method of transmitting data in packets on a communication network, said method comprising steps of compressing and decompressing the packets and, more particularly, the packet headers. In this method, the compression of the headers is performed by taking the difference between the previous packet and the current packet using error bit transmission. A receiving node is then responsible for detecting any errors in transmission. It then informs the intermediate nodes of any errors and these nodes then decide to compress or decompress the packets including the headers.
This method takes advantage of the temporal redundancy between two successive packets but may prove inefficient for certain configurations. What is more, the management of the transmission errors makes the method complicated to put into practice.